Analyzing refractive changes after anterior segment surgery

Abstract 

Purpose

To present a comprehensive method for analyzing astigmatism after anterior segment surgery and to demonstrate the applicability of this method in evaluating patients having laser in situ keratomileusis (LASIK).

Methods

Calculation of mean refractive power, refractive surgical effect, and preoperative, postoperative, and surgically induced astigmatism is presented using vectorial analysis and matrix formalism. Indices of refractive surgical success, that is, a global index of correction (GIC), a spherical index of correction (SIC), and astigmatic or surgical accuracy (SA), are presented. Interpretation of keratometric data and correlation with regular refractive astigmatism may require normalization of keratometric data, for which a method is provided.

Results

There was a significant difference between the mean preoperative (−7.16 +0.95 × 87.86) and postoperative (−0.66 +0.095 × 82) refractive powers (P < .001). The mean refractive surgical effect was 5.64 +0.84 × 178.51, with a mean preoperative (K1), postoperative (K3), and surgically induced (K2) astigmatism of 0.95 diopter (D), 0.095 D, and 0.84 D, respectively. For the refractive data, the mean GIC and SIC were 90% and 90%, respectively, with an SA of 0.75. There was a significant correlation between keratometric and refractive astigmatism (r = 0.52, P < .001) and between the change in the refraction and keratometry (r = 0.94, P < .001).

Conclusion

The method presented allows a meaningful interpretation of both refractive and keratometric data and calculation of mean refractive power without the need to change the format of the data from conventional notation. While the results pertain to 1 surgeon, LASIK had a greater effect in correcting the spherical component of refractive power than the astigmatic component. The correlation between refractive and normalized keratometric astigmatism indicates that the cornea is a main determinant of astigmatism. The high correlation between the change in myopia and keratometry after LASIK shows that although the cornea may not be the main determinant of myopia, the effect of LASIK in reducing myopia appears to be mediated through a change in corneal curvature.